Loudspeakers for reproducing high frequency range sounds are normally called tweeters. The DVD Audio, Super Audio, which are the audio equipment introduced recently to the market, reproduce the music sources whose frequency range has been extended to 20 kHz or even higher frequency. So, it is desired that the tweeters reproduce high frequency sounds higher than 20 kHz, preferably up to 100 kHz. At the same time, it is desired that loudspeakers be compact in size, in order to meet the generally prevailing trend in the field of audio equipment toward downsizing.
In the meantime, there has been a number of problems among the conventional tweeters with dome shaped diagrams in reproducing the sounds higher than 20 kHz.
To address a problem of decreasing driving force in the high frequency range, a modified structure in the tweeter, or a leaf tweeter, has been proposed.
A conventional leaf tweeter is described with reference to FIG. 9A–FIG. 9D, and FIG. 10. In FIG. 9A–FIG. 9D, and FIG. 10, diaphragm 23 is formed of film 20, coil 21 and frame 22, and magnetic circuit 29 comprises bottom yoke 24, outer yoke 25, plate 26, magnet 27 and two magnetic gaps 28 formed between outer surfaces of plate 26 and inner surfaces of outer yoke 25. Diaphragm 23 is disposed so that coil 21 is located on the upper level of magnetic gap 28, and frame 30 fixes diaphragm 23 and magnetic circuit 29. Conventionally, insulating cushion material 31 is provided between magnetic circuit 29 and diaphragm 23.
In the above-configured leaf tweeter, when electrical input is delivered to coil 21, a driving force is generated in coil 21 which has been integrated with film 20, and film 20 is driven by the driving force generated in coil 21 without loosing the driving force and film 20 radiates the sound waves. Thus the leaf tweeters are advantageous in reproducing sound waves higher than 20 kHz.
However, the above-described leaf tweeters have the following drawbacks:    (1) The width of magnetic gap 28 in leaf tweeters is several times as large as that of generally-used dome-diaphragm tweeters. This results in a decreased magnetic flux density. Furthermore, since the most concentrated magnetic flux in the magnetic gap 28 is not used, an efficiency of magnetic circuit 29 is low due to its structure.
Namely, in order to make the magnetic flux concentrated to magnetic gap 28, the conventional leaf tweeters are provided with plate 26 having a width at least the same as that of magnet 27, and outer yoke 25 equipped with a protrusion protruding towards magnetic gap 28, as shown in FIG. 9A–FIG. 9D. Furthermore, a magnetically saturated condition is created within outer yoke 25 for diffusing as much magnetic flux upward. The magnetic flux, however, diffuses also downward; therefore, it is difficult to collect the magnetic flux from magnet 27 efficiently upward to the upper level of magnetic gap 28, where coil 21 is disposed.    (2) As reproduction sound pressure of a loudspeaker increases in proportion to a magnetic flux density of magnetic gap 28, a size of magnet 27 has to be increased to make the sound pressure high. Larger magnet 27 naturally necessitates larger plate 26 fixed on the upper surface of magnet 27. Bottom yoke 24 and outer yoke 25 are also required to be larger. These lead to a larger size magnetic circuit 29, which does not meet with the recent requirement for compact loudspeakers.    (3) In the conventional loudspeakers, directions of magnetic flux at two magnetic gaps 28A and 28B shown in FIG. 10 are opposite to each other. Therefore, a direction of electric current is reversed between coils 21A and 21B. Although one portion provided for reversing the direction of electric current is utilized as a connection member for connecting with a lead wire, the opposite portion is not exposed to the magnetic flux. Therefore, this portion has remained as one of the elements that deteriorate efficiency of the coil. Thus, if a larger driving force is required in a loudspeaker of a conventional structure, it has been difficult to make the loudspeaker small and light in weight, as it inevitably necessitated an enlarged magnetic circuit.
The present invention addresses the above-described problems, and aims to provide an excellent loudspeaker that has a compact magnetic circuit yet can generate a sufficiently high sound pressure.